1. Technical Field
The present invention relates to a pulse generator circuit that generates pulses suitable for UWB (Ultra Wide Band) communication, and also relates to a communication apparatus.
2. Related Art
UWB communication is a high-speed, large-capacity data communication method using an ultra-wide frequency band. Other examples of the communication method using a wideband signal include conventional spectrum spreading and orthogonal frequency division multiplexing (OFDM). On the other hand, UWB is a wider band communication method using ultra-short pulses, and also called an impulse radio (IR) communication, which is hereinafter referred to as a UWB-IR method or simply referred to as an IR method. In the IR method, only time-base operation, which is not based on conventional modulation, allows modulation and demodulation, and it is expected that the circuitry is simplified and the power consumption is reduced (see U.S. Pat. No. 6,421,389, United States Patent Application Publication No. 2003/0,108,133 A1, and United States Patent Application Publication No. 2001/0,033,576).
The pulse waveform used in the IR method will be briefly described below with reference to the drawings. A pulse train having a pulse width PD and a cycle TP shown in FIG. 10A is well known, and the frequency spectrum of the pulse train is a sinc function the envelope of which has a first zero point at a frequency BW=1/PD, as shown in FIG. 10B.
The pulses shown in FIG. 10B are not easy to use because the spectrum spans DC to BW, whereas pulses shown in FIG. 10D, in which the carrier frequency f0 at the center of the spectrum is on the high frequency side, are preferable. The pulse waveform is obtained by clipping the portions of a rectangular wave having a frequency f0=1/(2PW) that correspond to the pulses in FIG. 10A and moving them toward the high frequency side of the frequency spectrum. The waveform, however, contains a DC component indicated by the dashed line 1601 in FIG. 10C, and does not have the ideal spectrum shown in FIG. 10D in the exact sense.
A waveform having the ideal spectrum is shown in FIG. 10E. The waveform is obtained by multiplying the pulses shown in FIG. 10A by a sinusoidal wave having the carrier frequency f0. FIG. 10F shows a waveform obtained by multiplying the pulses shown in FIG. 10A by a rectangular wave having the carrier frequency f0, and the waveform is readily generated by a digital circuit. Since an actual digital circuit has a narrow pulse width, such a square-cornered waveform will not be generated, but the waveform shown in FIG. 10E will be obtained. When the waveform shown in FIG. 10C is used to drive an antenna, the DC component will not be radiated from the antenna, and a signal the waveform of which is close to the waveform shown in FIG. 10F will be radiated.
Other pulse waveforms ideal for the UWB communication have been invented, and Gaussian pulses and Hermitian pulses are believed to be suitable. Although different from the waveforms shown in FIGS. 10A to 10G, these pulses are frequently used because they are readily generated.
In the UWB communication, the thus generated pulses are used not only in a transmitter, but also in a receiver as template pulses for evaluating correlation with a received signal. In the receiver, differential signal processing is often carried out, and two signals having phases inverted from each other shown in FIG. 10G are often required. The differential pulse signals are also effective in the transmitter, for example, when a balanced antenna is driven. In the receiver circuit, what is called IQ signals, in which the phase of the in-phase signal differs from the phase of the quadrature signal by 90 degrees, is further required in many cases.
For example, “A Low-Power Template Generator for Coherent Impulse-Radio Ultra Wide-Band Receivers,” Jose Luis et al, Proceedings IEEE ICUWB, 2006, pp. 97-102 presents a circuit for generating balanced pulses. The circuit, which has several differential delay circuits connected in tandem, generates a pulse train having a pulse width that corresponds to the amount of delay generated in a single delay circuit based on a logic circuit. “A Low-Power Template Generator for Coherent Impulse-Radio Ultra Wide-Band Receivers,” Jose Luis et al, Proceedings IEEE ICUWB, 2006, pp. 97-102 suggests the possibility of reduction in power consumption by carrying out a pulse startup operation both in rise and fall periods of a signal inputted to the delay circuits, as well as the possibility of IQ signal generation by using every other delay circuit.
The pulse generator circuit of the related art described above having a simple circuit configuration can accurately generate ultra-high-frequency, ultra-wideband pulses necessary for UWB communication, and the generated fine pulses are substantially circuit constituent device performance limited.
As described with reference to FIGS. 10A to 10G, however, the spectral characteristics of the pulses generated by the circuit of the related art follow a sinc function, and hence suffer from a significantly broad side lobe. To use the pulses generated by the circuit in communication applications, the pulses need to be band-limited in some way. In the related art, a filter is used to band-limit the pulses. A problem with the filter is that the filter should be a bandpass filter that works at an ultra-wideband, ultra-high frequency that is close to the device performance limit and has good skirt characteristics and such a configuration is not easy to achieve.